In this configuration, the primary winding and the feedback winding are arranged such that a sustaining oscillation is ensured once the supply is switched on. The waveform’s duty cycle is asymmetrical, but it is not very important in this application. Please note that if the oscillations do not occur at the ‘switch-on’ time, the transformer winding terminals of the feedback or the primary winding (but not both) should be reversed.

Source: http://www.electronic-circuits-diagrams.com/psimages/powersuppliesckt6.shtml